Amperometric continuous measurement system



s. A. HADDAD 2,851,655

AMPEROMETRIC CONTINUOUS MEASUREMENT SYSTEM Fil ed Sept. 25, 1956 Sept.9, 1958 CHLORINATED v ATER SUPPLY NEEDLE VALVE FIG.I

POTASSIUM CHLORIDE AND IODIN E \N WATER a: c a A E I ,(l l "I Y I I Q Il INVENTOR. R l A A SAMIR A. HADDAD BY I VOLTAGE 1 It AGENT United atesAMPEROMETRIC CONTINUOUS MEASUREMENT SYSTEM 2 Claims. (Cl. 324-30) Thisinvention relates to electrochemical measurements 7 wherein electrodesare used in a liquid, and has particularreference to the measurement ofelectrical current in a circuit which includes a liquid path betweensuch electrodes.

Various amperometric devices in electrochemical measurement systems havebeen used in the past but none have been fully satisfactory and nonehave provided, on a practical basis, for the continuous measurement ofthe concentration of a particular component in a liquid, especiallyunder conditions of potentiometric changes in the liquid solution whichis being measured.

The system of this invention may be used to measure residual chlorine inwater supplies in terms of such chlorine as translated into iodine formeasurement purposes, or for the measurement of other similarly suitableelectrolytically decomposable components.

This invention provides a new and useful system for the continuousmeasurement of the concentration of a preselected component of a liquidsolution by continuously flowing the liquid under measure through aconduit wherein a pair of electrodes are fixedly mounted in spacedrelation along said conduit. The electrodes are closely similar as tomaterial, size, and shape, and are mounted transversely of the conduitso that the flowing liquid under measure must pass through theelectrodes. The electrodes are formed with numerous uniformly spacedperforations therethrough for this purpose. An example of suitablematerial for such electrodes is platinum. Both electrodes may be ofplatinum or one may be of platinum and the other of gold or they may beof materials in any combination wherein, because of the selectedmaterifis, the electrodes are both, and similarly, subject to changewith respect to potential upon the occurrence of potentiometric changessuch as pH changes in the flowing solution under measure. Accordingly,the two electrode coinlii-ii-a-tionof this invention provides electrodepotential solution equilibrium with respect to potentiometric changes inthe solution. The fixed potential nature of such prior art devices ascalomel, silver, or silver chloride electrodes and the like make themunsuitable for use in the device of this invention, and systems usingsuch prior art electrodes require some other means of identifying andcompensating for potentiometric changes such as pH in the solution undermeasure. On the other hand, the system of this invention automaticallycompensates for such potentiometric changes.

It therefore, is an object of this invention to provide a new andimproved amperometric measurement system.

Other objects and advantages of this invention will be in part apparentand in part pointed out hereinafter and in the accompanying drawing,wherein:

Figure I is a schematic showing of an illustrative embodiment of thisinvention in the form of a residual chlor-ine concentration measurementwith respect to a water solution, wherein the electrolytically unstablechlorine is associated with potassium iodide to'provideelectrolyticalatent" 1y stable iodine in a water solution forconcentration meas urement in representation of the chlorineconcentration; and

Figure II is an amperometric curve illustration with respect to themeasurement of iodine (i. e., chlorine) concentration wherein the effectof a potentiometric change such as pHin the liquid under measure, isillustrated.

Figure I illustrates the application of an embodiment of this inventionto the measurement of chlorine con, centration in a Water solution.

A chlorinated water sample take-off pipe 10 is con nected to achlorinated water supply pipe 11 as a means of feeding a sample supplyto a sample tank 12 which is allowed to at least slightly overflowconstantly asv a means of maintaining a constant head of sample watersupply. From the water tank 12 an outlet pipe 13 carries the samplewater through a conventional needle valve and ball float system 14 formaintaining a constant flow in the chlorinated water supply and anoutput pipe 15 from the ball float unit feeds the chlorinated water intoa mixing container 16. On the other hand a reagent supply of potassiumiodide solution is provided in a tank 17. The outlet of this tank is apipe 18 leading to a secondconventional flow controlling needle valveand ballfloat unit 19 with a ball float outlet pipe 20 carrying thereagent into the mixing-container 16 so that the chlorinated water andthe potassium iodide are joined in the mixing container 16 to form awater solution of potassium chloride and iodine. The resultant potassiumchloride, as Well as any excess potassium iodide, is inert and will notdecompose under the electrochemical conditions of this device so thatthe remaining component of iodine is therefore measurable as toconcentration in representation of the chlorine concentration in thewater supply being tested.

From the mixing tank 16 the water solution of potassium chloride, iodineand excess potassium iodide, if any, is carried away by a pipe 21' to ameasuring conduit 22 wherein a pair of electrodes R and A are located.The iodine solution thereafter fiows to waste as indicated at 23 or itmay be salvaged as a means of reclaiming the component thereof in anysuitable conventional manner (not shown).

In the Figure I system, electrode R is a reference electrode, ordinarilyestablished at essentially zero potential, and electrode A is anindicator electrode, established at a potential, with respect to thereference electrode potential, which is sufficient to establish apotential range capable of encompassing the polarization current curveof the particular component being measured.

The electrodes R and A are mounted in fixed relation within the conduit22 and transversely perpendicular thereto. These electrodes are in theform of perforated discs with the perforations substantial in number anduniformly spaced throughout the sample flow facing faces of theelectrode discs. The sample solution thus flows through both discs whichby their spaced apart relation along the length of the measuring conduit22 make it possible for the sample solution to act as an electricalconductor in the provision of a current path between the electrodes Rand A. The diffusion layer forms not on the reference electrode R but onthe indicator electrode A. It is desirable for this diffusion layer tobe quite thin for quick response in the operation of this system andthis is accomplished by providing a sufficient flow of sample liquid toactually physically tear down the diffusion layer to a desired thinness.However, the diffusion layer must be effectively uniform as to thinnessthroughout the surface of the electrode which is exposed to the sampleflow. For this reason the electrode is formed as a screen.

Thus small unit area face portions thereof are presented to the sampleflow and thus the flow is past a substantial number of small, uniformlyarranged electrode areas. Each such area thus gets essentially the sameflow application and for practical purposes the combination of the rateof formation of diffusion layer and the rate of sample flow, maintain athin diffusion layer which is essentially uniform throughout the entireflow meeting face of the electrode.

The electrodes R and A, and when a device is in operation, the iodinesolution therebetween, form a part of an electrochemical measuringcircuit which includes a fixed resistor 24 and a variable resistor 25 inseries between the electrodes R and A. This circuit is energized from adirect current source such as battery 26 and the measurement of thecurrent in the circuit it taken, for example, across the resistor 24 asa voltage drop, by a suitable conventional measuring instrument 27.

The functioning of the system according to this invention is illustratedby the voltage-current curve showing of Figure II. The potentialsassigned and applied to the electrodes R and A in Figure I through thearrangement of their associated circuit and particularly throughadjustmcnt of the variable resistor 25, are important in that accordingto preselection and preknowledge under a given set of electrochemicalconditions the polarizing curve of a particular composition componentsuch as iodine would be initiated as to substantial current increase atone potential and would terminate as to substantial current increase atanother potential which is essentially the beginning of the limitingcurrent plateau of such a curve. The amplitude variation of thepolarizing curve such as that indicated by X in Figure II is determinedby the concentration of the component which is being measured. Whateverthe concentration and consequent amplitude of the curve such as curve X,the full curve nevertheless for a particular composition will fallwithin a potential range extending between the two electrode potentialvalues which are preselected according to prior knowledge, at least inapproximation. Thus in Figure II curve X represents one value ofconcentration of chlorine and curve X represents a greater concentrationvalue of chlorine. Note that in both cases the full curve lies betweenthe electrode potentials indicated in curve Figure 11 as R and A. Thisis the measurin operation which continues in this fashion with varyingamplitude as long as there is no change in the potentiometric conditionsof the chlorine solution. This means of course, the iodine solution asis actually measured. However, under conditions of potentiometric changeit is important that the polarizing curve still remains within the rangeof potential which lies between the potentials of the two electrodes. Itis a substantial advantage of this invention that when a potentiometricchange occurs in the solution being measured, the potentials which havebeen assigned to each of the two electrodes are both automaticallychanged. This is because of the nature of the material of the electrodeswhich have been selected and of the similarity of the electrodes fromelectrochemical and form and dimension standpoints. Thus as in Figure IIcurve Y represents the polarizing curve of the same concentration asthat indicated in curve X except that curve Y represents the situationwherein a potentiometric change has occurred in the solution undermeasure, for example, a pH change has occurred. Thus the potentials ofthe electrodes R and A now become new potentials at R and A and thepotential range or span between electrodes R and A is essentially tehesame as that new span between electrodes R and A. For this reason, themeasurement of the amplitude of the curve Y produces the same result asthe measurement of the amplitude of the curve X, that is, an automaticcompensation of potentiometric change in the solution has been carriedout. Without such automatic compensation, as would occur with prior artdevices, the range of polarity between the electrodes would not properlyspan the polarizing curve after a potentiometric change in the solution.For example, the potential at the electrode A might then fall, undersuch prior art conditions, at a point on the polarizing curve prior tothe full peak of amplitude of that curve or substantially after thebeginning of the limiting current plateau of that curve or potential ofthe electrode R might well no longer be coincident with the potential atwhich the polarizing curve starts to rapidly rise. Thus in order toproperly match up the electrode potentials with the polarizing curve itis necessary to have the automatic potentiometric change compensation ofthis invention.

This invention, therefore, provides a new and improved electrochemicalmeasurement device wherein amperometric measurements are made with a twoelectrode system wherein the relative potential relationship between theelectrodes is essentially maintained in an automatic compensation actionwith respect to potentiometric changes in the liquid solution 'beingmeasured. This device may be described as a fixed applied potentialsystem with an operational potential adjustment in compensation forpotentiometric changes such as pH changes in the liquid solution beingmeasured.

As many embodiments may be made of the above invention, and as changesmay be made in the embodiments set forth above, without departing fromthe scope of the invention, it is to be understood that all matterhereinbefore set forth or. shown in the accompanying drawings is to beinterpreted as illustrative only and not in a limiting sense.

1 claim:

1. An amperometric measurement system for continuously determining theconcentration of a preselected comonent of a fiOWlIlg liquid and forcontinuously compensating for potentiometric changes in said flowingliquid, wherein an electrical current is measured in relation topre-established potentials on a pair of metallic electrodes in saidflowing liquid, and wherein said continuous compensation maintains aneffectively fixed differential between said potentials to provide theeffect of electrical current measurement with respect to fixedpotentials on said electrodes, said system comprising, in combination, aconduit for carrying said flowing liquid, a fixed volume measurementunit formed of a length of said conduit with conduit filling end wallsin the form of said pair of metallic electrodes, said pair of electrodesbeing fixedly mounted in said conduit and thus in spaced relation witheach other lengthwise of said conduit, said electrodes beingcharacterized in that they are perforated discs essentially identical inshape and essentially equal in size, and with said perforations in theform of identical, multiple, uniformly spaced screen openings, in thatthey are mounted completely stationary in and transversely at rightangles to said conduit, whereby said flowing liquid meets and passesthrough said electrodes essentially at right angles thereto and becauseof said stationary right angled mounting and said multiple openings,only a thin, evenly distributed diffusion layer is maintained on one ofsaid electrodes, in that the materials of said electrodes arepreselected as subject to change with respect to electrical potentialthereon upon the occurrence of potentiometric changes in said flowingliquid, and in that said change characteristics are at least similar soas to provide, in combination with said identical shape, equal size, andidentical openings, the result that over a preselected range, anelectrical potential difference between said electrodes remainseffectively unchanged upon said occurrence of said potentiometricchanges, thus providing said automatic compensation for saidpotentiometric changes, a direct current electrical measurement circuitincluding said electrodes and the said flowing liquid therebetween, adirect current source, means including electrical resistance means forapplying dilferent electrical potentials to said electrodes from saiddirect current source with said electrode potentials preselected, andapplied by adjustment of said resistance means, with reference to theamperometric characteristics and curves of said preselected component inthat for a given electrochemical set of conditions the polarizingcurrent curve of said preselected component lies in the electricalpotential range between said preselected electrode potentials asrepresented by said fixed potential diflerential, and means formeasuring the concentration of said preselected component in terms offixed potential drop electrical current through said circuit, as variedonly by changes in said concentration of said preselected component insaid flowing liquid.

2. An amperometric measurement system for continuously determining theconcentration of residual chlorine in water in a situation whereinchlorinated water is mixed with potassium iodide as a reactant toproduce a flowing water solution of potassium chloride and iodine, withsaid measurement system applied to the measurement of the concentrationof said iodine as representative of the concentration of said chlorine,for continuously compens-ating for pH changes in said flowing liquid,wherein an electrical current is measured in relation to pre-establishedpotentials on a pair of platinum electrodes in said flowing liquid, andwherein said continuous compensation maintains an effectively fixeddifferential between said potentials to provide the eflect of electricalcurrent measurement with respect to fixed potentials on said electrodes,

said system comprising, in combination, a conduit for carrying saidflowing liquid, a fixed volume measurement unit formed of a length ofsaid conduit with conduit filling end walls in the form of said pair ofplatinum electrodes, said pair of electrodes being fixedly mounted insaid conduit and thus in spaced relation With each other lengthwise ofsaid conduit, said electrodes being characterized in that they areperforated discs essentially identical in shape and essentially equal insize, and with said perforations in the form of identical, multiple,uniformly spaced screen openings, in that they are mounted completelystationary in and transversely at right angles to said conduit, wherebysaid flowing solution meets and passes through said electrodesessentially at right angles thereto and because of said stationary rightangled mounting and said multiple openings, only a thin, evenlydistributed diffusion layer is maintained on one of said electrodes, inthat said platinum material of said electrodes is subject to change'with respect to electrical potential thereon upon the occurrence of pHchanges in said flowing solution, and in that said changecharacteristics are essentially identical so as to provide, incombination with said identical shape, equal size, and identicalopenings, the result that over a preselected range, an electricalpotential difference between said ele' trodes remains effectivelyunchanged upon said occurrence of said pH changes, thus providing saidautomatic compensation for said pH changes, a direct current electricalmeasurement circuit including said electrodes and the said fixed volumeof flowing liquid therebetween, a direct current source, means includingelectrical resistance means for applying diflerent electrical potentialsto said elec trodes from said direct current source with said electrodepotentials preselected, and applied by adjustment of said resistancemeans, with reference to the amperometric characteristics and curves ofsaid residual chlorine in that for a given electrochemical set ofconditions the polarizing curve of said residual chlorine lies in theelectrical potential range between said preselected electrode potentialsas represented by said fixed potential differential, and means formeasuring the concentration of said residual chlorine in terms of fixedpotential drop electrical current through said circuit, as varied onlyby changes in said concentration of said residual chlorine in saidflowing solution.

References Cited in the file of this patent UNITED STATES PATENTS1,217,365 Talley Feb. 27, 1917 2,122,364 Christie June 28, 19382,289,687 Stuart July 14, 1942 2,350,378 Wallace June 6, 1944 2,615,839Willier Oct. 28, 1952

